Changing RANKL/OPG mRNA expression in differentiating murine primary osteoblasts

Abstract

Osteoblast-osteoclast coordination is critical in the maintenance of skeletal integrity. The modulation of osteoclastogenesis by immature cells of the osteoblastic lineage is mediated through receptor activator of NFκB (RANK), its ligand RANKL, and osteoprotegerin (OPG), a natural decoy receptor for RANKL. Here, the expression of OPG and RANKL in primary mouse osteoblastic cultures was investigated to determine whether the osteoclastogenic stimulus depended on the stage of osteoblastic differentiation and the presence of the calciotrophic hormone 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). OPG mRNA expression was increased in osteoblastic cultures after the onset of mineralisation relative to less mature cultures, but did not alter in response to 1,25-(OH)2D3 treatment. In contrast, basal RANKL mRNA expression did not change during differentiation but was significantly enhanced by 1,25-(OH)2D3 treatment at all times. The stimulatory effects of 1,25-(OH)2D3 on RANKL were lessened in more mature cultures, however. The RANKL/OPG ratio, an index of osteoclastogenic stimulus, was therefore increased by 1,25-(OH)2D3 treatment at all stages of osteoblastic differentiation, but to a lesser degree in cultures after the onset of mineralisation. Thus the 1,25-(OH)2D3-driven increase in osteoclastogenic potential of immature osteoblasts appears to be mediated by increased RANKL mRNA expression, with mature osteoblasts having relatively decreased osteoclastogenic activity due to increased OPG mRNA expression. These findings suggest a possible mechanism for the recently proposed negative regulatory role of mature osteoblasts on osteoclastogenesis and indicate that the relative proportions of immature and mature osteoblasts in the local microenvironment may control the degree of resorption at each specific bone site.